Accepting that the aircraft turned from its intended course and commenced a descent from its assigned altitude would suggest deliberate crew action. The reasons for this may only be attributable either to unlawful intereference with flight or some technical malfunction(s). We are informed the former is most unlikely. This therefore must leave only the latter.
The nature of technical malfunction(s) must have involved a total loss of communications and in a short period of time thereafter escalated to limited control of the aircraft so as to prohibit further changes in course and altitude. The safe option under such circumstances would have been to keep clear of high terrain by remaining over water. At least dawn, by which time the crew may have become incapacitated leaving the aircraft to continue on its predermined course over the water, until its fuel was exhausted. Such actions would have taken the flight well beyond the areas within which SAR operations have commenced. No use looking in the wrong haystack for the needle.